DIFFERENTIATION AND MATURATION OF T CELLS IN THE THYMUS

1. DIFFERENTIATION AND MATURATION OF T CELLS IN THE THYMUS

2. BONE MARROW HSC HEMATOPOIETIC STEM CELL THYMUS LYMPHOID PRECURSOR MYELOID PRECURSOR BLOOD BLOOD B-cell NK-cell T-cell monocyte mast neutrophil DC TISSUES LYMPHOID TISSUES B-cell T-cell mackrophage mast neutrophil DC

3. T- CELL DEVELOPMENT Lymphoid precursor NK cell c-kit/CD44 B B B B RAG-1/RAG-2 Pro-T Pro-B -rearrangement -rearrangement Pre-T L rearrangement Surrogate L H rearrangement  Pre-T Pre-B T Selection clonal deletion Selection clonal deletion T T T Mature-T Mature-B

4. a a REGULATED T-CELL DIFFERENTIATION APC CD4+CD8+ TCR preT- Epithelial cell immatureTcell pre Tcell ANTIGEN RECOGNIZING RECEPTOR pro T cell SIGNALING RECEPTOR NO ANTIGEN RECOGNIZING RECEPTOR

5. EVENTS OF T CELL DIFFERENTIATION IN THE THYMUS Pro-T IL-7-dependent proliferation Early pre-T Pre-Tα-chain Lck signal β rearrangement unsuccesful β-chain γδ T-cell No selection Late pre-T CD4+CD8+ α rearrangement CD4+CD8+ αβ NKT-cell unsuccesful α-chain no positive selection negative selection αβCD4+ αβCD8+ • Generation of NK cells • – no TCR • 2. Differentiation of γδ and αβ TCR carrying T cells • 3. Selection of αβ TCR • – positive selection • – negative selection • 4. Differentiation of CD4+ and CD8+ T cell lineages

6. SELECTION OF T LYMPHOCYTES IN THE THYMUS AICD – Activation Induced Apoptosis PERIPHERAL TOLERANCE UNDER THE CAPSULE • The primary T cell pool is biased to MHC-specificity (V genes) 1-2% for one allotype • Focusing the T cell pool to self MHC recognition (+) • Elimination of useless clones • Elimination of self agressive clones (-) • CENTRAL TOLERANCE • Focusing The T cell repertoire for recognition of non self • Individualized T cell repertoire is available in the periphery • CD4 and CD8 co-stimulatory molecules are involved in positive selection IL-7-dependent proliferation CORTEX CD4-CD8- DN β+preTα TCRαβ CD4+CD8+ DP TCR(-) sMHC+sP sMHC+fP fMHC+fP selection CORTEX/ MEDULLA   NO  – selection MEDULLA – AICD αβTCR αβTCR CD4+CD8+

7. POSITIVE SELECTION OF DOUBLE POSITIVE (DP) T CELLS ALSO DIRECTS CD4 AND CD8 SINGLE POSITIVE (SP) T CELL COMMITMENT CD4+CD8+ CD4+CD8+ Thymic epithelial cell MHC-I + peptide complexes recruit CD8 MHC-II + peptide complexes recruit CD4 POSITIVE SELECTION FOR 3 – 4 DAYS, SUCCESSIVE α-GENE REARRANGEMENTS BARE LYMPHOCYTE SYNDROME (BLS) Lack of MHC class I – no CD8+ cells Lack of MHC class II – no CD4+ cells

8. SELECTION OF THE T CELL REPERTOIRE – CENTRAL TOLERANCE POSITIVE SELECTION – Thymic education (no instruction for specificity) Low avidity interaction of MHC - self peptide - TCR Thymic epithelial cells Self peptide composition and concentration (foreign peptides are not present) 80-90% of DN (CD4-CD8-) T cells is NOT positively selected PASSIVE CELL DEATH BY NEGLECTION NEGATIVE SELECTION – Central self tolerance High avidity of MHC - self peptide - TCR interaction Ubiquitous and abundant self antigens are present in the thymus Any thymic antigen presenting cell: epithelial cells, bone marrow-derived macrophages, dendritic cells THE GENERATION OF SELF MHC + FOREIGN PEPTIDE SPECIFIC T CELLS REQUIRES WEAK INTERACTION WITH SELF MHC + SELF PEPTIDE SELF RESTRICTED AND TOLERANT PERIPHERAL T CELL REPERTOIRE PHYSIOLOGICAL TRESHOLD

9. HOMEOSTASIS OF POSITIVE AND NEGATIVE SELECTION IN THE DEVELOPMENT OF THE AVAILABLE T LYMPHOCYTE REPERTOIRE Ratio of positive selection Homozygote Heterozygote Ratio of negative selection increases with the number of MHC genes Number of MHC molecules

10. IMMUNOLOGICAL MEMORY Inhabitants: 46 000 Area: 1 400 km2 • 1781: Measles epidemics in the Faroe islands • after the epidemics the island has remained measles free for 65 years • 1846: Another epidemics • Those, who were elder than 65 years and were sick in 1781 were not re-infected, but some elderly got sick • Life long protection against some viruses exists • Maintenance of memory does not require the sustained or intermitting presence of the virus

11. How antigen-specific Ab production is maintained? MODEL 2. MODEL 1. memory B cell plasma cell Bystander help: Cross-reactive antigens TLR ligands Cytokines... Memory B cells continuously differentiate into plasma cells Long-lived plasma cells in the bone marrow MODEL 3. • Repeated activation with the antigen drives B cell activation and plasma cell differentiation • role of follicular dendritic cells in antigen storage (months-years?) • Polio: reinfections with Sabin vaccine strain • subclinical infections (Diphteria in 10% of the population) • hidden antigens (Measles genes persist in neurons – can induce Subacute Sclerosing Panencephalitis)

12. T – CELL MEMORY Central Effector

13. NEGATIVE REGULATION OF IMMUNE RESPONSES AICD DIFFERENTIATION Naive lymphocytes Memory Primary effectors Secondary effectors Number of antigen specific cells EXPANSION AICD MEMORY

14. IMMUNOLOGICAL MEMORY – T CELLS Naive T cell Effector T cell cytokine production cytotoxicity Central Memory T cell Effector T cell Maintained by cytokines IL-7, IL-15 • previously activated, partly differentiated cell type • CCR7+, resides in the circulation (blood, lymphoid organs) • high proliferative ability upon activation • rapid differentiaton to effector stage Effector Memory T cell Effector T cell • previously activated, partly differentiated cell type • closest to effector stage • CCR7-, resides in blood and peripheral tissues • low proliferative ability, fast effector functions

15. Memory in acute infections Memory cells & specific antibodies can be detected without antigen for long time - does it correlate with protection? Vaccinia-specific memory B cells 0 10 20 30 40 50 years after vaccination Memory against cytopathic pathogens – pre-existing neutralizing Abs are important Protection against acute, lethal child-hood infections due to maternal antibodies through the placenta or breast milk Vaccines are succesful against such pathogens: Polio, Small pox, toxin-producing pathogens (Tetanus, Diphteria)

16. ART ART ART HIV CTL Th • Memory in persistent infections- INFECTION IMMUNITY • CONTROLLED INFECTION MAINTAINS CONTINUOUS GENERATION OF ANTIGEN-SPECIFIC EFFECTOR T CELLS AND ANTIBODIES • typical for non-cytopathic or weakly cytopathic persistent infection • no antigen-independent memory cells PATHOGEN ESCAPE IMMUNOPATHOLOGY peripheral solid organs lymphatic organs, hematogenic spread antigen dose time time HIV treatment interruptions Herpes simplex BCG vaccine strain HIV-2 Mycobacterium leprae Chronic HBV Malaria

17. a a APC APC APC APC APC APC CD8TCR CD8TCR CD8TCR CD4 TCR CD4 TCR CD4 TCR Ag Ag Ag T-CELL DIFFERENTIATION IN THE PERIPHERY Memory T-cell Activated T-cell Mature naiveT-cell